1. Field of the Invention
The present invention relates to a process for formation of a three-dimensional photonic crystal having a three-dimensional fine structure.
2. Description of the Related Art
The photonic crystal is a structure in which materials different in refractive index are periodically distributed. The refractive index difference between the constituting materials, and the periodicity in the structure give, as the most important characteristic of the photonic crystal, a photonic band gap, namely a region through which a specified electromagnetic wave cannot propagate. Introduction of an appropriate suitable defect into the refractive index distribution in the photonic crystal forms an energy level (defect level). Owing to this defect, the photonic crystal is capable of controlling an electromagnetic wave. Therefore, the photonic crystal is an artificial material which can realize a novel function readily by structure design. Moreover a device employing the photonic crystal can be far smaller than conventional devices.
A three-dimensional photonic crystal has three-dimensional periodicities of the refractive index of the constitution material, being less liable characteristically to cause leakage of an electromagnetic wave from the defect position. Therefore, the three-dimensional photonic crystal is the most suitable material for controlling electromagnetic wave propagation. Typical three-dimensional photonic crystals have a woodpile structure (or a rod-pile structure) as described in U.S. Pat. No. 5,335,240.
FIG. 7 illustrates a woodpile structure of the three-dimensional photonic crystal which is constituted of lamination of striped layers having respectively plural rods arranged parallel and periodically at prescribed in-plane periods. The rods in one striped layer (a first layer) cross perpendicularly the rods in an adjacent striped layer (a second layer), and are directed parallel to the rods in the striped layer (a third layer) next to the adjacent striped layer (the second layer) mentioned above with positional shift by half of the in-plane arrangement period. The rod-arrangement period in the photonic crystal structure is about a half of the wavelength to be controlled. For example, in the photonic crystal device for control of visible light, the in-plane arrangement period of the rods is about 250 nm.
Such a three-dimensional photonic crystal, although expected to have ideal device characteristics, has a complicated structure, and is produced usually through many complicated steps. For controlling a shorter wavelength of an electromagnetic wave, the required structural period should be smaller, and the critical dimension (CD) for the required structure should be smaller. This requires strict precision in the positional registration between the layers and in the structure processing.
A layer lamination technique for production of a three-dimensional photonic crystal is disclosed in Japanese Patent Laid-Open No. 2004-219688. In practicing this technique, firstly, on a striped layer formed on a substrate, a rod array is formed which contains parallel rods arranged at a prescribed in-plane arrangement period. Then two of the above striped-layers are joined by fusion bonding with positional registration. Then the substrate of the one-striped layer is removed. By repeating such an operation, a woodpile structure has layers in number corresponding to the number of the joining operations. By such a lamination technique, a three-dimensional photonic crystal having a relatively complicated structure can be produced.
Another production process of the three-dimensional photonic crystal is disclosed in APPLIED PHYSICS LETTERS 86, 011101 (2005). In this process, a three-dimensional photonic crystal is formed from silicon crystal by photoelectric chemical etching of a first face and FIB-drilling of a second face to remove a part of the silicon.
The three-dimensional photonic crystal, for achieving intended device characteristics, should have a dimension of prescribed number of the arrangement periods in the thickness direction as well as in the in-plane direction. Generally, the number of the arrangement periods in the thickness direction is 3 or more. Thus the above woodpile structure should have a lamination structure of 12 or more striped layers. Further, for achieving intended device characteristics, the processing error and the layer registration error should be made smaller.
In a woodpile structure of the three-dimensional photonic crystal, for example, the processing error of each of the rods is preferably not larger than about 10% of the rod arrangement period, and the positional registration error between the layers is preferably not larger than about 25% of the rod arrangement period. For a photonic crystal device for visible light, in which the in-plane rod arrangement period is about 250 nm, the rod processing error is not larger than about ±25 nm, and the layer registration error is not larger than about ±60 nm.
However, for production of the three-dimensional photonic crystal through a conventional lamination process, although a conventional semiconductor technique can be applied, the process is complicated, and the number of the production steps increases in proportion to the number of layers of the photonic crystal to increase technical difficulty and to lower the productivity. Moreover, positional registration of the layer should be conducted in each of the lamination operations, which will accumulate necessarily the registration errors. Further, at the interfaces between the layers, simultaneously with occurrence of discontinuity of the materials (or refractive index), dirt adhesion or contamination can occur unavoidably in the production process, causing undesired scattering of electromagnetic waves. Furthermore, increase of the number of the layers will increase stress in the structure to cause deformation of the structure. Such disturbances in the structure affect adversely the characteristics of the photonic crystal device.
In formation of three-dimensional photonic crystal from silicon crystal by a photoelectric chemical etching and FIB drilling as described in APPLIED PHYSICS LETTERS 86, 011101 (2005) (Non-Patent Document 2), the problems below arise.
Firstly, selection of the material of the base material is limited. When electrochemical etching is employed, the material should be selected which can be etched electrochemically, and the crystal face for the etching and the shape of the pores are also limited. Therefore, the flexibility in design and processing is low.
Secondly, in FIB drilling for formation of the three-dimensional photonic crystal, broken pieces of the base material sputtered by the ions can deposit again on the lateral walls of the pores unavoidably. Further, in the FIB drilling, a part of the ions are scattered and penetrates through the side walls of the pores into the base material of the photonic crystal to deteriorate the optical and electrical characteristics. Furthermore, the FIB which drills the pore one by one is not suitable for processing a large area, so that a large three-dimensional photonic crystal cannot readily be formed only by the FIB drilling at a low cost.
The present invention intends to provide a three-dimensional structure with high precision, in particular to provide a nano-photonic crystal structure, and a production process thereof at a lower production cost by solving the above problems.